But most thro' midnight streets I hearHow the youthful Harlot's curse (13-14)
In a poem this ominous, the word "midnight" is no good. It makes us think of darkness, death, and other bad things.
Blasts the new-born Infant's tear, And blights with plagues the Marriage hearse.(15-16)
The harlot's curse adds death to death. Marriage is already associated with death ("hearse") but being blighted with plagues only makes it more dead. Hmm, strange indeed. London isn't just dead; it's really, really dead.